This invention relates to the curing of films of photosensitized polyamic acid to form polyimides. More specifically, the invention relates to the photosensitization of polyamic acid by inclusion of a photoinitiator therein.
Polyimides are typically prepared by a condensation polymerization reaction between organic diamines and tetracarboxylic acid dianhydrides, and demonstrate excellent high temperature and dielectric properties. Because of the properties of polyimides, they have found broad utility in many areas.
Present commercial practice is to purchase polyimide films themselves or polyamic acid, the precursor of polyimides, in a liquid, and thus castable, form. The polyamic acid, once cast and the solvent removed, can be cured or imidized at elevated temperatures to the polyimide form. This curing temperature is in the range of 650.degree. F.
The problem with this practice is, of course, that the number of substrates available for application of a curable polyamic acid thereto becomes somewhat limited, i.e. they must be in essence temperature-resistant. Yet, in many instances, it would be desirable to use substrates in conjunction with a polyimide which would not be heat stable at the normal elevated curing temperatures for formation of the polyimide. Such applicability has not heretofore been available.
In commonly assigned and copending application Ser. No. 38,060 it is disclosed that ultraviolet radiation can contribute to the curing of films of polyamic acid to a polyimide. The condensation polymerization reaction involved in the curing process can therefore be undertaken at a much lower temperature than was normally thought necessary. This allows use of substrates, in conjunction with polyimides, which are not normally stable to excessive temperatures.
Furthermore, by use of the ultraviolet curing, polyamic acid films attain the status of being photodefinable, i.e. a mask can be selectively utilized to cure selected areas of the polyamic acid film in an imagewise fashion, thereby allowing removal of the unexposed portions, again in an imagewise fashion.
I have now discovered that by including photoinitiators in polyamic acid films, the curing reaction to polyimides becomes significantly faster under exposure to ultraviolet radiation.